The present invention relates to a temperature-dependent switch including a temperature-dependent switching mechanism comprising a snap-action disc, a housing which accommodates the switching mechanism and has a lower part and an upper part, two stationary contacts which are provided on an inner surface of the upper part, each stationary contact being connected to an associated outer connection, and also comprising a current transfer member which is arranged on the snap-action disc and can be moved by said snap-action disc, the snap-action disc pressing the current transfer member, in a temperature-dependent manner, against the two stationary contacts which serve as bearing areas for the current transfer member.
A switch of this kind is known from DE 198 27 113 C2.
The known switch comprises a housing with a cup-like lower part into which a temperature-dependent switching mechanism is inserted. The lower part is closed off by an upper part which is held on the lower part by the raised edge of the lower part. The lower part can be produced from metal or an insulating material, while the upper part is composed of insulating material or a PTC thermistor material.
Two contact rivets are situated in the upper part, the inner heads of said contact rivets serving as stationary contacts for the switching mechanism. The contact rivets project outwards though the upper part and turn into outer heads which serve as the outer connection of the known switch. Connection wires can be soldered directly onto these outer heads, it also being known to hold contact brackets with the outer heads, connection wires being soldered or crimped onto the said contact brackets.
The switching mechanism carries a current transfer member in the form of a contact plate, two mating contacts which are connected to one another being arranged on the upper surface of the said current transfer member and being brought into contact with the two stationary contacts depending on temperature, thereby electrically connecting the stationary contacts to one another. In this case, the stationary contacts serve as bearing areas for the contact plate.
The temperature-dependent switching mechanism comprises a bimetallic snap-action disc and also a snap-action spring washer, a pin which is fitted with the contact plate passing through the centres of the said bimetallic snap-action disc and snap-action spring washer. The snap-action spring washer is constrained circumferentially in the housing, while the bimetallic snap-action disc is supported on a shoulder of the lower part or on the edge of the snap-action spring washer depending on the temperature and, thereby, either enabling abutment of the contact plate at the two stationary contacts, or else lifting off the contact plate from the stationary contacts, with the result that the electrical connection between the outer connections is interrupted.
This temperature-dependent switch is used, in a known manner, to protect electrical appliances from overheating. For this purpose, the switch is connected electrically in series with the appliance to be protected and is arranged mechanically on the appliance such that it is thermally connected to the said appliance.
Below the response temperature of the bimetallic snap-action disc, the contact plate makes contact with the two stationary contacts, and therefore the electrical circuit is closed and the appliance to be protected is supplied with power via the switch. When the temperature increases beyond a permissible value, the bimetallic snap-action disc lifts the contact plate off from the stationary contacts, as a result of which the switch is opened and the supply to the appliance to be protected is interrupted.
The appliance which is now without power can then cool down again. In this case, the switch which is thermally coupled to the appliance also cools down again, the said switch then automatically closing again.
As a result of the dimensions of the contact plate, the known switch is able to carry much higher high operating currents compared to other temperature-dependent switches in which the operating current of the appliance to be protected flows directly across the bimetallic snap-action disc or a snap-action spring washer associated therewith, and therefore the switch can be used to protect relatively large electrical appliances with a high power consumption level.
As already mentioned, the known switch automatically switches on again after the appliance which is protected by it cools down. While switching behaviour of this kind may well be expedient for protecting, for example, a hairdryer, this is not desirable primarily in applications in which the appliance to be protected must not be automatically switched on again after having been switched off, in order to avoid damage. This is the case, for example, for electric motors which are used as drive assemblies.
DE 198 27 113 C2 therefore proposes a so-called self-holding resistor which is connected electrically parallel to the outer connections. When the switch is open, the self-holding resistor is connected electrically in series to the appliance to be protected, only a harmless residual current now flowing through the said appliance on account of the resistance value of the self-holding resistor. However, this residual current is high enough to heat the self-holding resistor to such an extent that it gives off heat which keeps the bimetallic snap-action disc above its switching temperature.
DE 198 27 113 C2 describes two different ways in which the self-holding resistor can be produced and fitted. In a first embodiment, resistor tracks are provided on the inner surface of the upper part, the said resistor tracks connecting the two stationary contacts to one another and carrying the residual current, which ensures self-holding, when the switch is open. In another embodiment, the upper part is produced from PTC thermistor material, and therefore the upper part itself forms the self-holding resistor.
Although the known switch has proven useful from a technical point of view, problems arise in the event of long-term use, particularly when very high currents are intended to be switched.
In order for two reproducible bearing areas which ensure secure contact and therefore low contact resistance to be provided for the current transfer member, the stationary contacts, that is to say the internal heads of the rivets, have to be symmetrical to the axis of symmetry of the upper part and current transfer member. Furthermore, the said stationary contacts have to be situated in one plane.
In order to achieve this, the heads of the contact rivets which are used in the known switch bear firmly against the upper part on the inside and on the outside. So-called contact brackets are then fixed to the outside of the upper part by way of the outer heads, connection wires being soldered or crimped onto the said contact brackets. This type of contact-making operation leads to secure contact between the current transfer member and the stationary contacts when the contact rivets are symmetrical to the axis of symmetry.
However, for production-related reasons, this symmetrical position is not always ensured. This is the case, for example, in upper parts which are composed of PTC thermistor material.
Within the scope of the present invention, a “PTC thermistor material” is understood to be a current-carrying ceramic material which has a positive temperature coefficient, with the result that the electrical resistance of the said ceramic material increases as the temperature increases. The temperature-dependent change of the electrical resistance value is not linear in this case.
PTC thermistors of this kind are also called PTC resistors. They are produced, for example, from semiconductive, polycrystalline ceramics such as BaTiO3.
In order to produce the PTC upper parts, mixtures of barium and titanium compounds and also other materials, which together exhibit the desired electrical and thermal properties, are compressed in a mould with the desired geometric dimensions and passage openings for the contact rivets, and are then sintered at high temperatures.
In this case, sintering can change the geometry of the upper parts such that the geometric position of the passage openings varies. Both the distance between the passage openings and the distance of the passage openings from the centre of the cover change in an unpredictable manner during sintering.
This leads to the current transfer member not always bearing securely on the stationary contacts, with the result that larger contact resistances than desired are produced.
These relatively large contact resistances lead, in particular with a high current flow, to heating of the contact rivets which expand, particularly in length, in such a way that the mechanical retention of the contact brackets is adversely affected and the contact resistance rises again. This, in turn, leads to further heating as a kind of positive feedback, this further heating further increasing the contact resistance, and so on.
This intrinsic current heating then leads to the switching temperature of the switch changing. Even when the temperature of the monitored appliance is below the response temperature of the switch, the additional intrinsic current heating can lead to the switch undesirably opening.
These problems occur particularly at high currents, for example in three-phase alternating current applications in which very high current flows in the individual phases. In this case, a temperature-dependent switch of the generic type is provided in each phase, said switch disconnecting the phase in the event of an impermissible increase in temperature.
In this case, it is desirable for all three phases to be disconnected simultaneously. However, this requires identical heat coupling for all three switches, it only being possible to achieve this with a great deal of technical outlay, if at all.
Therefore, it is also known to place a switch of the generic type in the star of a three-phase alternating current circuit by two phases being connected to the two outer heads of the contact rivets and the third phase being connected to the lower part which is produced from metal. The current in the third phase then flows through the snap-action disc and the pin, which mechanically connects the snap-action disc and the current transfer member to one another, into the current transfer member.
However, this solution is already unsatisfactory because the current flow through the snap-action disc likewise leads to intrinsic current heating, this likewise having an undesirable influence on the switching behaviour in the above-described sense.
Furthermore, in these designs, such high currents flow through the switch that the problem of lengthening of the contact rivets is even more pronounced.
In this connection it is known from U.S. Pat. No. 4,555,686 A to provide a temperature-dependent switch with a heater plate having arranged thereon three moveable contacts, each of which cooperates with a stationary contact. Said heater plate is being moved by a bimetallic spring, whereby constructional measures avoid that the heater plate can be rotated in its circumferential direction relative to the stationary contacts.
This switch is associated with the disadvantage that it is of complex design and that the permanent development of heat inside the switch makes it unsuited for guiding high currents.